Continuous particulate emissions monitor calibrator

ABSTRACT

A device (10) and method for monitoring particulate (22) flowing through a conduit (20) has a transmitter (16) and a receiver (18) positioned in optical alignment on opposite sides of the conduit (20). A light beam (50) is transmitted across conduit (20) from the transmitter (16) to the receiver (18). Particulate (22) flowing through conduit (20) interrupts light beam (50) causing a signal to be generated from which particulate concentration is determined. A calibrator assembly (40) has a motor (64) adapted to move a filter (100) having a selected percentage opacity within the path of light beam (50). Movement of the filter (100) modulates light beam (50) and a signal corresponding to the concentration associated with the percentage opacity is generated.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to a device for monitoringparticulates passing through a duct, such as for instance, passingthrough a duct in a baghouse discharge or from an industrial stack. Inparticular, the present invention relates to a device for monitoring thefrequency at which a light shown across an emissions duct isinterrupted. More specifically, the present invention is directed to amethod, system, and device for correlating a selected percentage opacitywith its associated reading in concentration units in connection withsuch an emissions monitor.

2. Description of the Related Art

Emissions monitoring has become increasingly important in response tostrict environmental regulations and increased public awareness ofenvironmentally-safe industrial processes. Numerous types of deviceshave been developed for monitoring particulate emissions in industrialapplications. In general, these devices monitor the particulate flowingthrough a duct or stack, and particularly, monitor the amount ofparticulate being emitted. In this regard, the greater the concentrationor percentage of particulate relative to the emissions as a whole, thegreater the quantity of pollutants entering the atmosphere. If anindustrial process emits pollutants into the air in excess of a maximumpermissible amount as set by the Environmental Regulators, great expenseassociated with fines and perhaps shut-down can be incurred.Accordingly, monitoring particulate emissions is extremely important formaintaining a clean environment and transacting business in accordancewith the law.

Numerous devices and systems have been developed for monitoringparticulate emissions. One conventional system utilizes what is calledopacity technology. In general, opacity devices shine light from atransmitter located on one side of a stack or duct to be monitored to areceiver located on the opposite side of the duct in optical alignmentwith the transmitter. As dust travels through the stack or duct, thedust both scatters and absorbs some of the light provided by the opacitydevice. By comparing the brightness or intensity of the light shiningacross the stack or duct when no emissions are occurring with the dimmerbrightness or intensity of light associated with dust traveling throughthe stack, a percentage opacity measurement can be obtained. Percentageopacity is a commonly used unit for measuring emissions. Another type ofemissions monitoring device, called an impaction or triboelectricdevice, utilizes an earth-grounded probe inserted into a stream ofparticles to be monitored. As each particle impinges onto the probe, atransfer of electrical charge occurs which results in an electricalcurrent at the probe. Monitoring the current results in a relativeemissions measurement.

The foregoing devices have numerous drawbacks which reduce theireffectiveness and desirability. For instance, opacity systems, which arebased upon the amount of light energy detected through passing dust,quickly become inaccurate as lenses used by the device become caked withdust. In other words, as particulates build up on the sensors, theopacity device is unable to distinguish between moving dust beingemitted from the stack or duct and stationary dust which continues tosettle on the sensors. Accordingly, the reading in such an environmentis inaccurate. In this regard, an opacity device having dust accumulatedon the sensors will show an emissions reading even when no emissions areoccurring. Accordingly, the sensors of an opacity device requireconstant cleaning. Similarly, impaction or triboelectric devices, whichhave a probe positioned within the dust stream, quickly become dirty andmust be repeatedly cleaned. Periodic cleaning of the foregoing devices,in addition to requiring repeated extensive time and effort, increasethe cost of using such devices.

A more recent device for monitoring particulate flowing through a ductor stack uses a DC light beam shining across the duct or stack to bemonitored. However, unlike the opacity devices which monitor lightenergy, these continuous particulate monitoring devices monitorinterruptions in the light beam caused by particulates passing throughthe light beam. In other words, as a particulate passes through thelight beam shining across the stack or duct, the light beam istemporarily broken. Accordingly, as particulate flows through the ductor stack, it passes through the light beam causing the light beam toflicker or modulate. A receiver for receiving the light beam monitorsthis flicker or modulation and a signal is generated for use incomputing the frequency of interruptions of the light beam. Theconcentration of particulate is proportional to the frequency ofmodulation. Since continuous particulate monitors of this type are notconcerned with the intensity of the light, but rather the interruptionof the light beam, particulate accumulated on the light transmitter orreceiver of the monitor does not reduce the effectiveness or accuracy ofthe monitor. Furthermore, it is known to increase the intensity of thelight beam proportional to the amount of stationary particulateaccumulated at the transmitter and receiver to insure consistent andaccurate particulate monitoring. Accordingly, a continuous particulatemonitor of this type for monitoring an interrupted light beam requirescleaning far less frequently than other conventional emissionsmonitoring devices. Moreover, as stated, unlike an opacity system, sucha monitor remains accurate despite particulates settling on the lightsensors. In fact, it has been found that such a monitor remainseffective even when over 90% of the transmitted light is blocked withparticulates, such as dust.

Despite the advantages of continuous particulate monitors utilizing aninterrupted light beam for monitoring, these devices, when calibratedwith an isokinetic test, provide an emissions measurement in units ofconcentration, not in units of percentage opacity. Conventionalpractice, particularly in the United States, is to rate emissions inpercentage opacity. Accordingly, the need exists for a continuousparticulate monitor, which utilizes light beam interruption, which canbe calibrated for percentage opacity readings. Particularly, the needexists for a calibration device, for use with emissions monitors usinglight beam interruption techniques, for calibrating the device forpercentage opacity readings. With such a device, the concentration ofparticulate flowing through a duct or stack relative to a selectedpercentage opacity reading could be determined. The present inventionprovides such a device and fills the foregoing and other needs.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a continuousparticulate monitor which is inexpensive and easy to manufacture, andwhich is easy to use and install.

It is a further object of the present invention to provide a continuousparticulate monitor, which utilizes light beam interruption, which canbe calibrated for percentage opacity readings.

It is a further object of the present invention to provide a calibrationor correlating device and method, for use with an emissions monitorusing light beam interruption techniques, for calibrating the monitorfor percentage opacity readings, and specifically, for correlating aselected percentage opacity with a concentration reading.

It is a further object of the present invention to provide a device andmethod for determining the concentration of particulate flowing througha duct or stack relative to a selected percentage opacity.

These and other objects are achieved by a continuous particulatemonitor, of a light beam interference type, having a calibrator assemblyfor correlating a known percentage opacity with its associated readingin concentration units. Particularly, the monitor has a transmitter anda receiver positioned in optical alignment on opposite sides of a ductor stack to be monitored. The transmitter transmits a beam of lightacross the duct or stack where it is received by the receiver. Asparticulates pass through the light beam, the light beam is interrupted.The transmitter and receiver are connected to processing hardware. Asignal indicative of the frequency at which the light beam isinterrupted by particulates is generated and sent to the processinghardware. The processing hardware includes a display for visuallydisplaying data indicative of the frequency at which the light beam isinterrupted. In other words, the modulation of the light signal in thestack or duct caused by passing particulates provides a basis fordetermining the concentration of particulates flowing through the stackor duct. Preferably, the hardware comprises a central processing unitfor calculating the concentration of particulates based upon knowledgeof the type of particulates flowing through the system and thedimensions of the stack or duct.

A calibrator assembly is positioned between either the transmitter orthe receiver and the stack or duct to be monitored. The calibratorassembly of the present invention is a device for calibrating theparticulate monitor for readings in percentage opacity. In this regard,the particulate monitor displays data indicative of the concentration ofparticulates flowing through the stack or duct being monitored. Since itis desirable to know the percentage opacity reading associated withparticulate flowing through the stack or duct, and particularly, sinceit is desirable to know when the emissions reach the permittedpercentage opacity limit, the present invention provides means forcalibrating the particulate monitor so that the particulateconcentration reading associated with a selected percentage opacity(e.g., the upper permitted opacity limit) can be determined.

The calibrator assembly has a calibrator body with a channel-aperturetherethrough. The aperture is placed in alignment with the light beamtransmitted from the transmitter and to the receiver so that light istransmitted through the aperture of the calibrator body. A channelextends from the upper, outer exterior of the calibrator body to thecentral aperture of the calibrator body. The channel is adapted toreceive a filter. The filter is connected to a filter holder assembly,which is in turn connected to a motor. When assembled, the filter ispositioned within the channel in alignment with the calibrator aperturesuch that any light passing through the aperture must also pass throughthe filter. When the motor is activated, the filter moves substantiallyupwardly and downwardly in a portion of the space defined by the channelin the calibrator, although the filter itself remains in completecoverage of the central aperture of the calibrator.

In use, a filter having a known opacity is placed into the channel inalignment with the central aperture of the calibrator body. To calibratethe monitor for a percentage opacity reading, the monitor is reset tozero, and the calibrator motor is turned on. As a result, the filter isplaced in motion. Once the filter is in motion, the receiver perceivesthe filter as particulates flowing through the dust or stack. A readingin accordance with the amount of particulate detected is therebyobtained. Since the percentage opacity of the filter is known, theconcentration reading obtained is equivalent to that known, selectedpercentage opacity. As a result, the operator now has knowledge of theconcentration reading corresponding to the percentage opacity of thefilter selected.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the invention noted above are explained inmore detail with reference to the drawings, in which like referencenumerals denote like elements, and in which:

FIG. 1 is a schematic view of a continuous particulate monitor withcalibration assembly, with the present invention positioned on a stackor duct for monitoring particulate flowing therethrough;

FIG. 2 is a top plan view of a portion of the particulate monitor of thepresent invention, and particularly, of a motor platform of thecalibrator assembly of the present invention;

FIG. 3 is a cross-sectional view taken along lines 3--3 of FIG. 2;

FIG. 4 is a cross-sectional view taken along lines 4--4 of FIG. 3; and

FIG. 5 is a cross-sectional view of the calibrator assembly of thepresent invention for illustrating movement of the filter.

DETAILED DESCRIPTION OF THE INVENTION

With reference initially to FIG. 1, a continuous particulate monitor ofthe present invention is denoted generally by reference numeral 10.Monitor 10 has connected thereto by wires 12, 14, a transmitter 16 and areceiver 18, respectively. As shown, transmitter 16 and receiver 18 areplaced in optical alignment with each other on opposite sides of a stackor duct 20. It will be appreciated that stack or duct 20 comprises aconduit for transmitting particulates resulting from industrialprocesses. For instance, stack 20 may be an industrial smoke stack forpassing emissions into the atmosphere. Alternatively, duct 20 maycomprise a duct in an industrial process, such as a baghouse dischargeduct. Furthermore, it should be appreciated that the conduit may take onshapes other than the cylindrical shape shown. Particulates 22 are shownflowing through the duct 20.

As shown in FIG. 1, duct 20 has a pair of pipes 24, 26 extendingoutwardly therefrom. Each pipe has a respective flange 28, 30 positionedat its outermost end. Prior particulate monitors of a type for sensinglight beam interference, such as those marketed under product line CPMby BHA Group, Inc., Kansas City, Mo., provide for coupling a transmitter16 and receiver 18 directly to respective flanges 28, 30. For instance,as shown in FIG. 1, continuous particulate monitor 10 of the presentinvention, which preferably utilizes such a CPM device by BHA Group,Inc., is shown with a flange 34 of receiver 18 coupled by bolts 32 toflange 30 of pipe 26.

In accordance with the principles of the present invention, transmitter16 has a flange 36 adjacent a calibrator body 38 of a calibratorassembly 40 of the present invention. Preferably positioned betweencalibrator body 38 and flange 28 are one or more high-temperatureinsulators, such as the pair of temperature insulators 42.

As shown in FIG. 1, calibrator assembly 40 is comprised of a calibratorbody 38 and a weather cap 44 which is bolted by bolts 46 to calibratorbody 38. Transmitter 16, calibrator body 38, and insulators 42 areconnected with flange 28 by bolts 48. It will be appreciated by thoseskilled in the art that insulators 42 are optional, and that additionalinsulators could be provided between the flange 30 of pipe 26 and theflange 36 of receiver 18. Additionally, it should be understood thatcalibrator assembly 40 may be positioned at the transmitter 16 as shownin FIG. 1, or alternatively, may be positioned at the receiver 18.Principles of operation of calibrator assembly 40 are not dependent uponits location.

It is seen that light transmitted from transmitter 16 shines throughapertures positioned through calibrator body 38, insulators 42, andflange 28 of pipe 24. A light beam 50 shines across the stack or duct 20where it is received by receiver 18. Monitor 10 is provided with akeypad 52, display 54, and further operating controls 56.

In accordance with the general operation of continuous particulatemonitor 10, light beam 50 is transmitted across duct 20 from transmitter16 to receiver 18. As particulates 50, traveling through duct 20,interrupt light beam 50, receiver 18 detects the interruption of thelight beam, and generates a signal indicative of the frequency ofinterruption of light beam 50. The signal is sent to monitor 10. It willbe appreciated by those skilled in the art that monitor 10, such asthose manufactured under the CPM product line by BHA Group, Inc.,provide means for then displaying on display 54 data indicative of thefrequency of light beam interruption. From this data, the concentrationof particulates flowing through the duct 20 can be determined.Alternatively, monitor 10 preferably comprises a central processing unitfor calculating particulate concentration utilizing the data receivedfrom receiver 18. In this regard, upon installation and start-up ofmonitor 10, an actual sample of the particulates flowing through duct 20is taken and analyzed for making various determinations, such as theconcentration of particulates flowing through the duct 20, and the typeof particulates flowing through duct 20. The actual particulateconcentration thereby obtained is compared with the data provided by themonitor 10, and monitor 10 is adjusted if necessary to the actual valueof the concentration obtained from the sample. As a result, monitor 10is thereby accurately calibrated for measuring the concentration ofparticulate flowing through duct 20.

It should be appreciated that the foregoing description, with theexception of the calibrator assembly 40, specifies the generallypreferred components for a particulate monitor of the type for detectinglight beam interference. With reference now to FIGS. 2-5, additionalcomponents and operation of calibrator assembly 40 is discussed.

FIG. 2 shows an enlarged top-plan view of the calibrator assembly 40 ofthe present invention, with weather plate 44 (FIG. 1) removed, and motorplatform and assembly 58 mounted by bolts 46 in its place. Withreference also to FIG. 3, motor platform and assembly 58 comprises aplatform 60 having a slot 62 formed therein. Slot 62 provides access tochannel 63 formed in calibrator body 38. A motor 64 is mounted to abracket 66 by bolts 68. Bracket 66 is connected to platform 60 by bolts70. A battery pack 72 and an on-off switch 74 are connected by wires 76to motor 64. A low-voltage indicator light 78 is connected by wires 80to battery pack 72 for indicating when the voltage supplied by batterypack 72 falls below a preselected level.

Motor 64 preferably provides a rotating output at a preselected RPMrating. Rod member 82 extends outwardly from motor 64, through anaperture (not shown) in brackets 66. Upon activation of motor 64, rodmember 82 rotates. Eccentric 84 is connected to rod member 82 by a pinconnection 86. Eccentric 84 has an outwardly extending pin member 88which is threaded through an aperture (not shown) in an upper portion ofarm 90 of filter holder assembly 92. A cap 94 is positioned on theoutermost end of pin member 88 for maintaining filter holder assembly 92in place.

Arm 90 of filter holder assembly 92 has a pair of prongs 96 at itslowermost end. Prongs 96 grip filter holder 98 having a filter 100positioned therein. A pin is threaded through prongs 96 and a small pinreceiving aperture in an upper central portion of filter holder 98. Itwill be appreciated that filter 100 is made from glass or plastic.Filter 100 has an outer rim 106. Filter holder 98 has a plurality oflocking tabs 108 for engaging the outer rim 106 of filter 100 forholding filter 100 in place. In this regard, it will be appreciated thatfilter 100 merely snap fits into filter holder 98.

As shown in FIGS. 2-5, grooves are provided at the ends of channel 63 ofcalibrator body 38 to provide guide rails 110 at the outer edges ofchannel 63. In this way, filter holder 98 fits substantially snug withinthe grooves of channel 63 of calibrator body 38, and is guided by guiderails 110 during operation of the device.

As shown in FIGS. 4 and 5, filter holder 98 is preferably provided witha pair of balancing apertures 112. Balancing apertures 112 reduce theweight of filter holder 98 in a balanced fashion. Further, filter holder98 is preferably more narrow at its uppermost end to provide a grippingarea 114 for prongs 96 of arm 93.

In operation, monitor 10 monitors particulates flowing through stack orduct 20. This operation is described in detail above, and it should beunderstood that a light beam transmitted across stack or ducts 20 fromtransmitter 16 to receiver 18 is interrupted by passing particulate.Receiver 18 generates a signal indicative of the frequency of modulationof the light beam caused by the passing particulates, and monitor 10provides data indicative of the concentration of particulate passingthrough duct or stack 20. In this regard, the concentration reading isin any preferred units, such as for instance, grams per cubic meter.Operation of the present invention for calibrating monitor 10 for apercentage opacity reading is now described.

To calibrate monitor 10 for a selected percentage opacity reading,weather cap 44 is removed from calibrator body 38 by removing bolts 46.In its place, motor platform and assembly 58, complete with filterholder assembly 92, is bolted into place by bolts 46. Motor platform andassembly 58 is positioned such that slot 62 in motor platform 60 alignswith channel 63 extending into calibrator body 38. Filter holderassembly 92 is positioned such that arm 90 extends from its connectingpoint on pin 88 of eccentric 84 through slot 62 of platform 60 and intochannel 63 of calibrator body 38. The components of the presentinvention are dimensioned such that filter 100 is positioned inalignment, and extends about the periphery of, aperture 65 in calibratorbody 38.

Motor 64 is activated by turning on-off switch 74 to the on position.When activated, motor 64 provides a rotating output, at a ratedrevolutions per minute, at rod member 82. As a result, eccentric 84rotates. As a result, arm 90, which is connected to pin member 88positioned near a peripheral edge of eccentric 84, travels about theperiphery along with its pin connection point 88. This causes the arm 90to reciprocatingly stroke generally upwardly and downwardly, therebycausing the entire filter holder assembly 92, and particularly thefilter holder 98 to move upwardly and downwardly within channel 63 ofcalibrator body 38. Movement of filter holder 98 is illustrated in thismanner in FIGS. 4 and 5.

As shown in FIG. 4, arm 90 is at the lower-most portion of its stroke aspin member 88 on eccentric 84 is at its lower-most position. In such aposition, filter 100; although remaining in a position such that itentirely blocks the channel-aperture 65 of calibrator body 38, isdisplaced downwardly in relation to the channel aperture 65. Bycontrast, as shown in FIG. 5, as arm 90 reaches the upper-most positionof its stroke, filter holder 98 has been drawn upwardly such that filter100 is displaced upwardly in relation to channel-aperture 65 ofcalibrator body 38. However, as shown in FIG. 5, filter 100 is stillpositioned such that it blocks the entire channel aperture 65 ofcalibrator body 38.

To properly calibrate monitor 10, filter 100 is preselected based uponits known percentage opacity rating. Although it should be appreciatedthat any percentage opacity filter could be selected, it is preferred toselect a filter having a percentage opacity rating equivalent to theupper percentage opacity limit of permissible emissions from the systemin which the present invention is utilized. Accordingly, for instance,if a particular emissions system is permitted to emit up to 20% opacity,a 20% opacity filter may be selected. A reading at monitor 10 takenthrough the moving filter 100, although in concentration units, will beequivalent to the opacity rating of the selected filter. As a result, inthis example, the concentration reading corresponding to 20% opacity isobtained, and the future use of the device can monitor for thatthreshold limit. It will be appreciated that data associated with thethreshold limit can be used for triggering alarms, or shut downcircuitry, which may be used with monitor 10.

It will be appreciated that numerous possible embodiments and variationsmay be accomplished without departing from the spirit and scope of thepresent invention. In this regard, it is necessary only that the filter100 is positioned within the light beam 50 between the transmitter 16and the receiver 18. Accordingly, its precise positioning within thatlight beam 50 is not critical. Additionally, although it is necessaryfor the filter to remain in motion at a rate >1.5 m/sec for propercalibration, it will be appreciated that numerous possible devices,including various types of motor assemblies, can be provided for movingfilter 100 and maintaining filter 100 in motion.

As discussed above, it is necessary for filter 100 to remain in motionfor proper calibration since, if filter 100 were maintained static,receiver 18 would perceive the filter as accumulated dust on thetransmitter or receiver. Accordingly, monitor 10 would respond by merelyincreasing the power to transmitter 16 or increasing the brightness oflight beam 50. Since monitor 10 senses only moving particulates, it isnecessary for filter 100 to be in motion for monitor 10 to sense thatwhich is blocking the light beam. In this regard, it should beunderstood that filter 100, which is obviously tinted to a selectedpercentage opacity, is viewed by the receiver as a multitude of veryfine particulates moving through duct or stack 20.

In accordance with the principles of the present invention, it has beenfound that it is necessary for filter 100, during the calibrationprocedure, to move in excess of 1.5 meters per second. It is importantto maintain the filter movement at greater than 1.5 meters per second toensure that the receiver senses moving particulate. If the filter ismoved any slower than 1.5 meters per second, the receiver will no longersense motion, but rather, monitor 10 will perceive the filter asparticulate accumulation on the transmitter 16 or receiver 18.Accordingly, to ensure that a proper speed filter movement ismaintained, motor 66 and the length of the necessary stroke of arm 90,keeping in mind the dimensions of filter 100 and channel aperture 65,are selected so as to ensure that movement of the filter is in excess of1.5 meters per second. Additionally, by utilizing the ratings of themotor, traditional voltage monitoring circuitry (not shown) is coupledwith the battery pack 72. When the battery drops below a voltagethreshold equated with the minimum RPM's necessary for maintainingmovement of filter 100 at greater than 1.5 meters per second, indicatorlight 78 emits light to visually indicate that the battery pack 72 needsto be replaced. It will be readily understood that other types of powersupplies, other than battery pack 72, may be utilized.

In the preferred embodiment, a 12-volt DC motor with a rotating outputup to 1,000 RPM's, such as manufactured by Philips, of Belgium issupplied with three DC volts. The visible portion of filter 100 (i.e.,within outer rim 106) in such an embodiment preferably has a 1.625"diameter, channel aperture 63 has a diameter of 1", and the central-mostportion of channel aperture 63 is positioned 3.813 inches from an axisextending from rotating output member 82 of motor 64. Eccentric 84preferably has a diameter of 0.88", and pin member 88 is preferablydisplaced 0.313" from the central axis thereof. Accordingly, one fullstroke of arm 90 is 0.626", or approximately 5/8 inches.

Additionally, although not shown in the drawing for illustrativereasons, it will be appreciated that selected components of the presentinvention, such as for instance, the motor, the battery pack, and thefilter holder assembly may be retained in a housing.

Additionally, it should be understood that the emissions need not beceased to calibrate the monitor. In this regard, as emissions arepassing through the duct or stack to be monitored, monitor 10 ispreferably reset to a zero reading. At this point, calibrator assembly40 is activated and the concentration reading at the display of monitor10 is taken. It is known that this reading corresponds to the percentageopacity of the filter utilized in the calibrator assembly 40.Alternatively, the step of resetting monitor 10 to zero can be omitted,calibrator assembly 40 can be activated, and the concentration readingassociated with the selected percentage opacity filter can be calculatedfrom the difference in readings with, and without, the calibratorassembly 40 in operation.

From the foregoing it will be seen that this invention is one welladapted to attain all ends and objects hereinabove set forth togetherwith the other advantages which are obvious and which are inherent tothe structure.

It will be understood that certain features and subcombinations are ofutility and may be employed without reference to other features andsubcombinations. This is contemplated by and is within the scope of theclaims.

Since many possible embodiments may be made of the invention withoutdeparting from the scope thereof, it is to be understood that all matterherein set forth or shown in the accompanying drawings is to beinterpreted as illustrative, and not in a limiting sense.

We claim:
 1. A device for monitoring particulates flowing through aconduit, said device comprising: means for shining a beam of lightacross the conduit; means for detecting modulation of said light beamcaused by particulates passing through said light beam, including meansfor generating a signal indicative of the concentration of particulateflowing through said conduit; means for correlating a selectedpercentage opacity with its associated reading in concentration units;and a filter having a selected percentage opacity positioned in opticalalignment with said light beam; and means for reciprocatingly movingsaid filter to provide modulation to said light beam.
 2. The device asset forth in claim 1, said correlating means further comprises: a filterassembly for engaging said filter, wherein said means for moving saidfilter moves said filter assembly with said filter engaged therein; anda body member for guiding said filter assembly during movement of saidfilter assembly.
 3. The device as set forth in claim 2 wherein said bodymember has a first face and a second face and an aperture extendingthrough said body from said first face to said second face, said bodyfurther having a channel extending from an outer surface of said body tointercommunicate with said aperture, whereby said filter assembly ispositioned within said channel such that said filter is aligned withsaid aperture extending through said body.
 4. The device as set forth inclaim 1, said moving means further comprising means to move said filterat a speed in excess of 1.5 meters per second.
 5. The device as setforth in claim 4 further comprising means for indicating when the speedof said filter falls below a preselected level.
 6. In an apparatus formonitoring particulate flowing through a conduit, said apparatusproviding a light beam across said conduit and comprising a sensor forsensing modulation of the light beam caused by particulate passingthrough the light beam, a device for correlating a selected percentageopacity with its associated reading in concentration units, said devicecomprising: a filter having a selected percentage opacity positionedsuch that said light beam passes through said filter; and means forreciprocatingly moving said filter such that said sensor sensesmodulation of said light beam caused by said reciprocatingly movingfilter.
 7. The device as set forth in claim 6 further comprising: afilter assembly for engaging said filter, wherein said means for movingsaid filter moves said filter assembly with said filter engaged therein;and a body member for guiding said filter assembly during movement ofsaid filter assembly.
 8. The device as set forth in claim 7 wherein saidbody member has a first face and a second face and an aperture extendingthrough said body from said first face to said second face, said bodyfurther having a channel extending from an outer surface of said body tointercommunicate with said aperture, whereby said filter assembly ispositioned within said channel such that said filter is aligned withsaid aperture extending through said body.
 9. The device as set forth inclaim 7, said moving means further comprising means to move said filterat a speed in excess of 1.5 meters per second.
 10. The device as setforth in claim 9 further comprising means for indicating when the speedof said filter falls below a preselected level.
 11. A method forcorrelating a selected percentage opacity with its associatedconcentration reading in a device which monitors particulate flowingthrough a conduit by detecting modulation of a light beam caused byparticulate interrupting the light beam, said method comprising thesteps of: placing a filter with a selected percentage opacity inalignment with said light beam so that said light beam passes throughsaid filter; reciprocatingly moving said filter so that said filtermodulates said light beam; and sensing the modulation of the light beamcaused by said filter and determining therefrom the associatedconcentration reading.
 12. The method as set forth in claim 11 furthercomprising the step of comparing said associated concentration readingwith said selected percentage opacity.
 13. The method as set forth inclaim 12 further comprising the step of resetting said device to a zerosetting prior to performing the step of moving said filter.